The present invention relates to a roll mill which allows a flat sheet to be continuously rolled mainly transversally or in the direction of its width. The present invention also relates to a rolling method which uses said roll mill to produce a flat sheet or a sheet with a special cross section having one or more longitudinal grooves of a desired shape on one side.
Sheets with special cross sections are conventionally produced by cutting, rolling or the combination of V-shaped dies and plain surface rollers. From the viewpoints of productivity and cost, the rolling method is considered to be most advantageous and many techniques have been proposed to implement this method. They are roughly divided into two types: according to the first type, a flat sheet is worked with a pair of rolls one of which has a projection that corresponds to the shape of the groove which is to be formed in the final product; in the second type of technique, a train of rolls having projections of slightly varying widths are employed, with inclined or arced reduction surfaces being provided on the lateral edges of each projection, and the width of a groove on the flat sheet is gradually increased by feeding it in the direction in which the width of projections increases. The groove formed by the first approach has a tendency to become undulate because of the difference in the degree of working as between the groove and other portions of the sheet. In order to avoid this problem, a comparatively thick sheet must be subjected to gradually increasing amounts of draft and this requires an increased number of passes and intermediate heat treatments. In addition, deep grooves cannot be formed by this method. A further disadvantage results from the fact that high rolling loads necessitate large equipment. In the second approach, reduction is taken only on the lateral sides of a groove and a sufficient amount of flow deformation occurs in the direction of the width of the sheet to minimize the formation of an undulate groove. Another advantage is the development of low loads during rolling operations. However, this method has the disadvantage of complicated operations since the sheet must be worked with a number of rolls having projections of different widths. This problem could be avoided by performing continuous operation on rolls arranged in tandem but in this case, it is essential to control the speed or tension at which the sheet travels from one roll stand to another. This requires not only the appropriate control devices but also a separate drive mechanism for each stand, which inevitably leads to the use of large equipment.